A wide variety of rigid metal prostheses, such as bone plates, intramedullary rods and femoral nails, are used in the fixation of bone fractures. A potential problem associated with the use of rigid bone prostheses for fracture fixation is referred to in the art as stress-shielding. As bone remodeling takes place in the region of the fracture, stresses exerted on the healing bone are carried primarily by the prosthesis rather than by the bone in the fracture region. This stress-shielding can be the cause of significant bone resorption, with consequent reduction of strength of the bone in the region of the healed fracture. Shielding of bending stresses from bone undergoing remodeling is believed to be particularly deleterious. See Bradley, G. W. et al., "Effects of Flexural Rigidity of Plates on Bone Healing", Jour. Bone and Joint Surg., Vol. 61-A, No. 6, pp. 866-872 (September 1979) and Woo, S. L-Y et al., "A Comparison of Cortical Bone Atrophy Secondary to Fixation with Plates with Large Differences in Bending Stiffness", Jour. Bone and Joint Surg., Vol. 58-A, No. 2, pp. 190-195 (March 1976).
The use of bone prostheses made of materials that are substantially less rigid than conventional surgical implant alloys has been proposed in order to alleviate problems arising from stress-shielding. One class of materials of reduced rigidity consists of non-absorbable synthetic polymers reinforced with inorganic fibers. However, the use of such materials sacrifices the excellent initial stabilization provided by a rigid prosthesis, which insures the maintenance of a proper alignment of bone segments during the early stages of healing.
U.S. Pat. No. 2,987,062 discloses an orthopedic bone fracture clamp adapted to be wrapped around a fractured bone. The clamp comprises two metallic bands directly connected at one pair of ends and joined by a link of absorbable catgut at the other pair of ends. The assembly of bands and absorbable link is placed under tension so as to hold the fractured bone together. After implantation of the assembly in the patient the link begins to be absorbed. Eventually the link fails and the clamping pressure on the bone is terminated, thereby, according to the patent, obviating the deleterious effects of continued pressure on the bone and eliminating the need for a second surgical operation to remove the clamp. However, the clamping pressure applied to the healing bone would not tend to decrease gradually or progressively with time after implantation. Instead, it would tend to drop in one full step at the moment of failure of the link from near the initial clamping pressure to zero.
The use of a bone plate having a rigidity that gradually decreases with time after implantation has been proposed by Parsons, J. R. et al., "A Variable Stiffness, Absorbable Bone Plate", presented at the Fifth Annual Meeting of the Society for Biomaterials, Clemson, South Carolina, April 28-May 1, 1979. The bone plate proposed by Parsons et al. is made of a material consisting of continuous carbon fibers embedded in a resorbable matrix of polylactic acid polymer. However, the initial rigidity of this plate is only 20% to 50% that of conventional stainless steel bone plates, and the rigidity decreases by only 10% after six weeks of implantation. Also, the carbon fibers are dispersed throughout the tissue of the patient after absorption of the polylactic acid matrix.